U.S. Pat. No. 4,291,959 discloses a photographic apparatus which is provided with two photocells, one for measuring ambient light and the other for measuring flash light, and further provided with two ND filters disposed respectively in front of the photocells to restrict the amount of light incident on the photocells. The ratio of the densities of the ND filters is changed in accordance with a diaphragm aperture size which, in turn, is determined in accordance with the measurement of the ambient light. The exposure time is determined in accordance with the integration of the output of the ambient light measuring photocell while the amount of light to be emitted from the flash device is determined in accordance with the integration of the output of the flash light measuring photocell. Thus, the ratio of exposures by the flash light and the ambient light is changed in dependence upon the ambient light level since the ratio of the densities of the ND filters is determined in dependence upon the ambient light level.
U.S. Pat. No. 4,317,620 discloses a photographic exposure and flash control system wherein an analog output of a photo sensor for detecting the light having passed through a camera diaphragm and incident on the sensor aperture is first converted into a digital signal. The value of the digitized data is limited to a minimum value if the brightness of the photographic object exceeds a predetermined first value and is limited to a maximum value if the brightness of the photographic object is below a predetermined second value lower than the first value. The output of the photo sensor begins, to be integrated upon commencement of the exposure of a photographic film, until the integrated value reaches a value corresponding to an exposure level less than, and having a given relationship with, the correct exposure level when the diaphragm aperture begins to close. When the actual aperture size reaches a desired value corresponding to the set or measured photographing distance, the flash device emits flash light corresponding to the digitized value.
U.S. Pat. No. 4,297,016 discloses an exposure control system, wherein the output of a photo sensor receiving and measuring the ambient light is integrated and the flash device emits flash light when the integrated value reaches a predetermined first value. The output of the photo sensor receiving the light reflected from the photographic object illuminated by the flash light as well as the ambient light is further integrated. When the integrated value reaches a second predetermined level, the shutter begin to close. The flash light emission is stopped when the integrated value reaches a third predetermined value. If the value, of the first integration of the ambient light does not reach the first level even after a lapse of a given period of time, the flash device starts after the predetermined time has passed. In this system, if the brightness of the ambient light exceeds a given value, the ratio of the amount of the flash light to that of ambient light that contributes to the exposure is fixed, but if the brightness of the ambient light is lower than the given value, the ratio of the amount of the flash light changes in accordance with the brightness of the ambient light from the fixed value up to 100%.
All of the prior art systems described above can only be used in such type of photographic cameras having a diaphragm aperture for the photo sensor besides the camera diaphragm aperture so that the photo sensor receives the light from the photographic object passed through the sensor-assigned aperture during the photographic film exposure.
Therefore, the prior art systems described above cannot be used in the lens interchangeable photographic camera such as the single lens reflex cameras, wherein the exposure time and actual aperture size are determined before the commencement of the exposure operation on the basis of the output of the photo sensor which is adapted to receive the light from the photographic subject having passed through the photographing aperture.
Another disadvantage of the prior art systems is in that exposure information to be used in the camera can not be displayed before the photography begins.
Furthermore, the prior art systems described above are directed to be applied to the lens shutter type camera with which flash photography can be effected at any exposure time. In other words the prior art systems described above are not suitable for a focal plane shutter camera, since the exposure time for the flash photography must be lower than a predetermined range to avoid a slit exposure.
The photographic apparatus shown in U.S. Pat. No. 4,291,959 has a disadvantage that it requires two photo sensors and two kinds of ND filters.
The exposure control system shown in U.S. Pat. No. 4,297,016 requires two kinds of filters for the ambient light and the flash light and an interchange mechanism for interchanging the filters.
U.S. Pat. No. 4,188,104 discloses an automatic control device for electronic flash which enables the correct exposure for either the main photographic object and the sub photographic object. In this device, the exposure time and the desired aperture value are calculated by a photographing distance data of the main object, a photographic distance data of the sub photographing object, the brightness data of both of the photographic subjects and the amount of the flash light emitted by the flash device, so that the exposure control is performed by the calculated exposure time value and the desired aperture value.
In this device, many kinds of data must be inputted, in other words, many sensors to obtain the input data must be provided. Furthermore, this device is composed of analog circuits with linear characteristics, requiring a wide information dynamic range so that this device is not suitable for a photographic camera system. Another problem in this device is in that where the brightness of the ambient light is relatively low, the actual aperture size must be large, so that the depth of focus becomes small causing a difficulty in the focus adjustment, thereby resulting in increasing the possibility of out-of-focus photographs.
Furthermore in a case where the brightness of the ambient light is relatively low, the fill-in flash mode is seldom used, so that the device shown in U.S. Pat. No. 4,188,104 is not so effective in spite of the complicated circuit arrangement.
Besides the prior art systems described above, in many conventional photographic cameras, the actual aperture size can be set to a predetermined value independent of the brightness of the photographic object. In using such a conventional photographic camera, even if the scene is suitable for fill-in flash photographing, only a simple flash light photographing is available, thereby causing the sub photographic object to be overexposed.